1. Field of the Invention
The present invention relates to electroluminescent (hereinafter referred to as EL) devices which are used in various instruments of, for example, emissive-type segment displays and matrix displays, or in displays in various information terminal appliances. The present invention also relates to methods for producing the same.
2. Related Arts
Conventional EL devices are formed by laminating a first electrode made of an optically-transparent ITO (indium tin oxide) film, a first insulating layer comprising Ta.sub.2 O.sub.5 (tantalum pentoxide) or the like, a luminescent layer, a second insulating layer, and a second electrode made of an ITO film, on an insulating glass substrate in that order.
The luminescent layer comprises, for example, a host material such as ZnS (zinc sulfide) with a luminescent center such as Mn (manganese) or Tb (terbium) added thereto, or a host material such as SrS (strontium sulfide) with a luminescent center such as Ce (cerium) added thereto.
EL devices give different colors depending on the choice of the additives in ZnS. For example, those having Mn as the luminescent center produce yellowish orange colors, while those having Tb produce green colors. EL devices having Ce as the luminescent center in SrS produce bluish green colors.
To realize full-color EL displays, luminescent layers capable of producing red, green and blue colors must be formed. Of these, SrS with a luminescent center of Ce is generally used as the material for the blue-emitting layers in EL devices. However, this material naturally produces a bluish green color. Therefore, in order to attain pure blue emission, a filter capable of cutting off the green component out of the emission spectrum must be used.
As opposed to this, it is known that an EL device with a CaGa.sub.2 S.sub.4 :Ce luminescent layer having Ce as the luminescent center element added to the host CaGa.sub.2 S.sub.4 (calcium thiogallate) can produce a blue color without using a filter, for example, as so reported in SID 93 Digest, pp. 761-764 (1993).
However, it is reported in the reference that the CIE (Commission Internationale de l'Eclairage) chromaticity coordinates for the EL device with the CaGa.sub.2 S.sub.4 :Ce luminescent layer indicate x=0.15 and y=0.19. On the other hand, the CIE chromaticity coordinates for ZnS:Ag that is used for the blue phosphor in cathode-ray tubes indicate X=0.15 and y=0.07 or so. The purity of the blue color produced by the conventional CaGa.sub.2 S.sub.4 :Ce luminescent layer is low and therefore the luminescent layer was unsatisfactory.